Secure of safety is one of requirements which are indispensably needed for industrial facilities as well as mechanical equipment used in a daily life. For example, a robot used in automated industrial sites employs a variety of safety units and devices for reducing or preventing the risk of a safety accident within a hemispherical work envelope of the robot.
Such safety devices can be classified into an active control type and a passive control type depending on a scheme of performing a safety function. The active control-type safety device senses an external impact and actuates an actuator in response to the detected external impact so as to cope with the external impact. For example, when a robot collides with an external object, a sensor mounted in the robot senses whether or not the robot crashes into the external object and its collision strength and transfers the sensed information to a controller which in turn generates a control signal in response to a signal indicative of whether or not the robot crashes into the external object and its collision strength so as to drive the actuator to cope with the external impact. The passive control-type safety device copes with an external impact applied thereto on a basis of a buffer mechanical element (shock absorbing mechanical element) such as a spring or a damper, but does not adopt a sensor or a separate actuator.
The active control-type safety device entails a merit in that it can actively cope with a change in an external condition. Nevertheless, the impact between a robot and an actual object, for example, since most impacts between a robot arm and a worker occur within approximately 15 to 20 ms, an active control-consuming time through the sensor, the controller and the actuator is greater than the physical time, thus causing a limitation in an impact-absorbing effect by the active control type.
On the other hand, the passive control-type device entails a merit in that it does not require a sensor or a separate actuator, thereby reducing a manufacturing cost and absorbs an external impact using the shock absorbing mechanical element such as a spring or a damper, leading an increase in response speed and no risk of an erroneous operation. Nevertheless, the passive control-type device has a disadvantage that it is difficult to implement a desired non-linear stable control operation. For example, in case where a passive control-type safety device such as a spring is mounted at a joint of the robot as a safety device for use in the robot, there occurs shortcomings that the spring is deformed in proportion to even a daily external force that does not require the absorption of an external impact as well as the robot arm is drooped in proportion to the weight of an external object exerted to the spring, which makes it impossible to a desired operation through the robot arm.